Unprotected sexual intercourse is the predominant risk factor for acquiring HIV, with most transmission occurring from infected men to male and female partners.1 The efficiency of HIV transmission depends on a variety of factors, including the type of sex act, the viral burden of the infected partner, and the susceptibility of the uninfected partner to HIV infection.2-7 Although behavioral interventions, such as condom use, have been successful in reducing transmission,8-10 approximately 4 million adults and children are infected globally on a yearly basis.1 New and novel prevention methods are urgently needed to slow the spread of the epidemic.
One potential mechanism to slow HIV transmission is the use of antiretroviral drugs (ARVs) to reduce HIV RNA concentrations in infectious secretions. ARV therapy has been shown reliably to decrease HIV RNA concentrations in the genital tract (GT),11-14 and lower semen HIV RNA concentrations are expected to decrease the efficiency of sexual transmission. Chakraborty and colleagues4 predict that a semen HIV RNA concentration of 100,000 copies/mL would result in transmission in 1 per 100 episodes of heterosexual intercourse, whereas a seminal viral load of 1000 copies/mL would decrease the probability of transmission to 3 per 10,000 acts. The relation between HIV RNA concentrations in blood plasma (BP) and semen is imperfect,15-18 and careful examination of viral sequences demonstrates that BP and GT can be viewed as separate viral compartments.19-21 Persistent GT HIV RNA shedding in subjects receiving ARVs has been reported,22 and long-lived resistant variants in the GT represent a particular problem for rebound viremia23 and transmitted resistance.19 It seems likely that poor penetration or altered metabolism of ARVs in the GT contribute to this problem.24
Nucleoside reverse transcriptase inhibitors (NRTIs) such as lamivudine (3TC) and zidovudine (ZDV) form the backbone of a typical ARV regimen, and this combination is currently recommended as an alternative option for initial treatment of HIV infection by the US Department of Health and Human Services.25 Random BP and seminal plasma (SP) concentrations of these drugs have been measured previously,14,26,27 and investigators have noted ZDV and 3TC concentrations to be 2 to 9 times higher in SP than in BP. As with all NRTIs, however, the active moiety is the triphosphate (TP) metabolite formed by intracellular enzymatic processes.28 Higher intracellular 3TC-TP and ZDV-TP concentrations in peripheral blood mononuclear cells (PBMCs) have been correlated with a faster decline in HIV RNA concentrations and an increase in CD4 T-cell counts.29 TP concentrations have not been previously characterized in GT mononuclear cells but are likely critical for NRTI efficacy locally. Here, we report on a comprehensive evaluation of extracellular and intracellular ZDV and 3TC concentrations in the male GT over a 12-hour dosing interval under steady-state conditions.
Study Design and Population
HIV-infected adult male subjects receiving ZDV/3TC twice daily as part of their ARV regimen were enrolled in a nonblind, open-label, descriptive pharmacokinetic (PK) study to assess the relation between parent drug concentration, intracellular drug metabolite concentrations, and HIV-1 RNA concentrations in BP and SP from May 2000 to June 2003. Subjects were recruited from the Infectious Disease Clinic at the University of North Carolina at Chapel Hill. Exclusion criteria included: age ≤17 years; vasectomy; active bacterial, fungal, or opportunistic GT or systemic infection at the time of enrollment; unwilling or unable to donate semen; abnormal blood chemistries or blood cell counts; or unable to receive ZDV/3TC for any reason. This protocol was approved by the Biomedical Institutional Review Board of the University of North Carolina at Chapel Hill.
In addition to 300 mg of ZDV and 150 mg of 3TC dosed twice daily, subjects could receive protease inhibitors or nonnucleoside reverse transcriptase inhibitors as part of the ARV regimen prescribed by their clinic provider. Subjects received the study regimen for 4 to 48 months before protocol enrollment. Before the PK visit, subjects completed a 7-day dosing card to ensure steady-state conditions; for the visit to continue, subjects must have achieved at least 90% adherence overall and have taken all doses in the preceding 3 days. Subjects were then admitted to the Verne S. Caviness General Clinical Research Center for an overnight stay for intense BP sampling and 2 semen samples. Matching blood and semen samples were also obtained at 3 outpatient visits over 2 weeks to complete the PK sampling from the seminal compartment and to measure HIV-1 RNA response in BP and SP.
Sample Collection and Processing
At the steady-state PK visit, blood samples were obtained immediately before the next dose (time = 0), and at 0.5, 1, 2, 4, 6, 8, and l2 hours after an observed dose of ZDV/3TC. Semen samples were obtained by masturbation immediately before dosing at time = 0, and again 12 hours after dosing. Paired BP and SP samples were obtained at 3 outpatient visits over 2 weeks at 3, 6, and 9 hours after a dose of ZDV/3TC for extracellular concentrations and intracellular TP concentrations. The time interval between semen collections was designed to avoid potential effects of repeated sampling on drug concentrations in this compartment. Subsequent formal study of the effect of sampling interval of ARV concentrations in semen demonstrates that short sampling intervals have no significant effects on measured concentrations.30
Whole blood was obtained using ethylenediaminetetraacetic acid (EDTA)-containing collection tubes (BD Diagnostics, Franklin Lakes, NJ) and was centrifuged at 1400 g (2800 rpm) for 15 minutes at 4°C. The resultant BP was aliquoted into labeled cryovials and stored at −80°C until analysis of extracellular ZDV/3TC concentrations and HIV-1 RNA concentrations. PBMCs were obtained using 10-mL cell preparation tube (CPT) tubes with sodium citrate (BD Diagnostics) for intracellular ZDV/3TC concentration analyses. After collection and centrifugation at 300 g (1300 rpm) for 30 minutes, the resultant plasma and buffy coat were transferred to a 15-mL Falcon tube and washed with phosphate-buffered saline (PBS). After an additional centrifugation step, the cell pellet was resuspended and viable PBMCs were counted with a hemocytometer using the trypan blue exclusion method. After the cells were repelleted and lysed with methanol, the cellular debris was removed in a final centrifugation step and the methanolic supernatant containing the TP was transferred to a labeled cryovial and stored at −80°C until analyses.
Semen samples were collected into a standard specimen collection cup. After collection, samples were left at room temperature to liquefy for 45 minutes and then centrifuged at 1400 g (2800 rpm) for 15 minutes at room temperature to separate SP from the cellular fraction. SP aliquots were transferred to labeled cryovials and stored at −80°C until analysis of extracellular ZDV/3TC concentrations and HIV-1 RNA concentrations. To isolate seminal mononuclear cells, the cell pellet was resuspended and layered on a Percoll gradient before centrifugation at 1400 g (2800 rpm) for 20 minutes. The cells were removed from the interface, washed with PBS, and resuspended. Cells were then counted and processed for storage using the same method as described previously for PBMCs.
Extracellular and intracellular concentrations in blood and semen were measured using validated and previously published liquid chromatography tandem mass spectrometry (LC/MS/MS) methods.31,32 The dynamic range for extracellular concentrations of ZDV and 3TC was 5 to 5000 ng/mL, with a minimum of 90% accuracy, and interday and intraday variability of 3% to 14% and 1% to 7.6% relative standard deviation (RSD), respectively. The lower limits of quantitation (LLQs) for intracellular ZDV-TP and 3TC-TP were 0.10 and 10.5 pmol, respectively. For concentrations up to 150 pmol, recovery was ≥93%, and the coefficient of variation (CV) for interday variability ranged from 4.5% to 8.4%. The CV for intraday variability ranged from 1% to 8%.
HIV-1 RNA concentrations in BP were determined using the Roche Amplicor Monitor kit (Pleasanton, CA; LLQ = 50 copies/mL) and in SP using the Organon Teknika Nuclisens assay (Durham, NC; LLQ = 400 copies/mL).
PK parameters, including the area under the time-concentration curve over the 12- hour dosing interval (AUC0-12h), were estimated for BP and SP using WinNonlin Professional (Version 4.1; Pharsight, Inc., Mountain View, CA). Half-lives were calculated from λz values obtained from the noncompartmental analysis. For all analyses, concentration measurements lower than the lower limit of detection were imputed as 0 and those lower than the LLQ were imputed as ½ LLQ. Concentration ratios were calculated to describe differences between drug exposure in BP and SP for extracellular ZDV and 3TC and intracellular ZDV-TP and 3TC-TP.
Demographic data for the 14 men enrolled in the study are presented in Table 1. These men ranged in age from 26 to 48 years; 50% were African American and 50% were white. In addition to ZDV/3TC, most (64%) were also receiving a protease inhibitor or a nonnucleoside reverse transcriptase inhibitor. At baseline, the median HIV RNA concentration in BP was 2.6 log10 copies/mL and <400 copies/mL in SP; the median CD4 T-cell count was 512 cells/mm3. Men were followed in this study for 2 to 44 weeks. Over this period, HIV RNA levels declined to undetectable concentrations in blood and semen in all but 1 participant. This subject had been receiving a stable regimen of ZDV/3TC for 3.5 years before enrollment and had undetectable HIV RNA in BP up to 32 weeks after enrollment, with a single detectable HIV RNA measurement of 880 copies/mL at 2 weeks of enrollment.
Table 2 presents median (interquartile range [IQR]) AUC0-12h in SP and BP for ZDV and 3TC and their intracellular TP metabolites. Five of 112 total ZDV BP samples were lower than the LLQ, and imputed values were used for these samples; all other samples were within the quantitation limits for each assay. For ZDV, the AUC0-12h in SP was 3790 (IQR: 2481 to 4783) h·ng/mL, whereas the AUC0-12h in BP was 1479 (IQR: 1175 to 1748) h·ng/mL. By dividing each subject's SP AUC0-12h by their respective BP AUC0-12h, the SP/BP exposure ratio was calculated. Using this AUC ratio approach, rather than using the SP/BP ratios of concentrations obtained at individual time points, provided a robust overall estimate of GT drug penetration. For ZDV, the median (IQR) SP/BP AUC0-12h ratio was 2.28 (1.48 to 2.97), indicating that ZDV exposure in SP was twice that of BP. For 3TC, the median (IQR) SP/BP AUC0-12h was 6.67 (4.10 to 9.14), indicating that 3TC exposure in SP was >6 times that of BP.
In Figure 1A, the median (IQR) time-concentration profile for ZDV depicts higher exposures in SP than in BP over the dosing interval. The maximal concentration (Cmax) observed in each compartment was similar (median SP Cmax = 561 ng/mL, median BP Cmax = 785 ng/mL). Using the available data, the median ZDV half-lives in SP and BP were 6.9 hours and 2.7 hours, respectively.
Figure 1B illustrates 3TC concentrations in SP consistently higher than in BP throughout the dosing interval. Generally, SP concentrations remained constant across the dosing interval. The median 3TC half-lives in SP and BP were calculated to be 7.1 hours and 3.8 hours, respectively.
Figure 2 depicts the median (IQR) TP PK in blood and semen for ZDV-TP (see Fig. 2A) and 3TC-TP (see Fig. 2B). These concentrations were more consistent across the dosing interval than extracellular ZDV and 3TC. For ZDV-TP, the PBMC and seminal mononuclear cell median (IQR) half-lives were 25.0 (5.5 to 28.5) and 14.2 (12.9 to 14.3) hours, respectively. For 3TC-TP, PBMCs and seminal mononuclear cell median (IQR) half-lives were 10.4 (3.3 to 23.1) and 10.2 (1.2 to 22.2) hours, respectively.
For ZDV-TP, the median (IQR) AUC0-12h in SP was 646 (361 to 1029) h·fmol/106 cells, whereas the median (IQR) AUC0-12h in BP was 1460 (1241 to 2172) h·fmol/106 cells. The SP/BP ratio for ZDV-TP was 0.36 (IQR: 0.30 to 0.37). For 3TC-TP, the median (IQR) AUC0-12h in SP was 82,068 (64,342 to 139,404) h·fmol/106 cells, whereas the AUC0-12h in BP was 108,600 (IQR: 78,687 to 164,984) h·fmol/106 cells, yielding an SP/BP ratio of 1.0 (IQR: 0.62 to 1.30). Although extracellular GT concentrations were higher than BP, the intracellular TP concentrations in seminal mononuclear cells were similar to, or lower than, PBMC concentrations. For ZDV-TP, the SP/BP AUC0-12h of 0.36 indicates that the seminal cell exposure was approximately 40% that of PBMCs. The SP/BP AUC0-12h ratio of 1.0 for 3TC-TP indicates equivalent exposure, however.
Figure 3 compares SP/BP concentration ratios at each collection point for the 4 analytes. The concentration ratios for ZDV-TP and 3TC-TP did not differ appreciably over time, whereas they were highly dependent on the time of sampling for parent ZDV and 3TC. For ZDV and 3TC, individual SP/BP concentration ratios ranged from 1.9 to 91.4 and from 1.9 to 30.5, respectively, with CVs ranging from 60% to 285%. In comparison, parent drug AUC0-12h ratio CVs ranged from 48% to 87%. Samples obtained early in the dosing interval gave lower SP/BP ratios, whereas those obtained later in the dosing interval gave higher ratios. This is particularly evident for ZDV, which maintains high concentrations in SP over a dosing interval, whereas BP concentrations decline quickly. For ZDV-TP and 3TC-TP, individual SP/BP concentration ratios ranged from 0.11 to 2.9 and from 0.14 to 2.5, respectively.
Correlation analysis to investigate potential relations between extracellular parent drug and the TP metabolite exposures in SP and BP did not provide strong evidence of a direct relation for ZDV or 3TC (r < 0.6, P > 0.07 for all analyses; data not shown).
This is the first study designed to provide full extracellular and intracellular PK profiles in blood and semen for ZDV and 3TC. Using calculated AUCs in this investigation, ZDV SP exposure was approximately 2 times greater than BP exposure, and 3TC SP exposure was approximately 6 times greater than BP exposure. Three previous reports of ZDV and 3TC concentrations in SP using isolated time points reported ZDV and 3TC SP exposures ranging from 2.16- to 9.1-fold greater than BP exposures.14,26,27 By obtaining several timed samples over 2 weeks, this investigation was able to calculate an AUC0-12h in SP, which, when compared with the AUC0-12h in BP, provides a more accurate estimate of drug penetration over a dosing interval.
Different ARV exposures in the GT relative to BP have been reported for men and women,14,24,33-40 and the exact physiochemical properties that govern drug passage into GT secretions are currently unknown. Differences exist between classes of agents and between specific agents within each class. This behavior in the male GT is consistent with steady-state GT behavior in women (3TC 4-fold higher in GT, ZDV 2-fold higher in GT).24
Similar to our previous report for the nonnucleoside agent efavirenz,33 extracellular ZDV and 3TC individual SP/BP concentration ratios change considerably in a single subject over the dosing interval because of different rates of drug penetration into the GT. Therefore, the use of AUC0-12h ratios provided a more robust measure of overall drug exposure in SP relative to BP. This can be seen by the lower CVs for AUC0-12h ratios (range: 48% to 87%) compared with individual concentration ratios (range: 60% to 285%). Given the logistics of seminal sample collection for constructing an AUC0-12h, however, there is some increased variability around the estimates of individual PK parameters (particularly Cmax).
Unexpectedly, this investigation found that elevated extracellular ZDV and 3TC exposures in semen did not result in elevated intracellular TP exposures. Although 3TC exposure was approximately 6-fold higher in semen than in blood, 3TC-TP exposure in seminal mononuclear cells was similar to that in PBMCs. Despite 2-fold higher ZDV exposure in SP compared with BP, ZDV-TP exposure in seminal mononuclear cells was only approximately 40% of that measured in PBMCs.
Our subjects were patients on therapy for HIV infection, and as such, sampling was limited to within a dosing interval. Although this approach can underestimate half-life, the long intracellular half-life estimates of ZDV-TP and 3TC-TP reported here (median values of 10.2 to 25.0 hours) are similar to those reported elsewhere in PBMCs.32,41,42 The more stable TP concentrations observed over the dosing interval suggest that a single SP/BP concentration ratio may serve as a reasonable proxy for overall intracellular drug exposure in the male GT. Because collection, processing, and analysis of PBMCs and seminal mononuclear cells are complex procedures, using single time points rather than multiple sampling strategies could facilitate research in this area.
The differential TP exposures between PBMCs and seminal mononuclear cells observed here may be attributable to differential cellular activation or different cell populations between the 2 compartments. 3TC (a cytidine analogue) and ZDV (a thymidine analogue) have different intracellular metabolic pathways for activation.28 The enzymes involved in these pathways can be affected by cellular factors, such as growth phase and activity.28 ZDV is preferentially phosphorylated in activated cells,43,44 whereas 3TC is preferentially phosphorylated in quiescent cells.45 These subjects had been on stable therapy for at least 6 months and had been screened for sexually transmitted diseases (STDs) before participating in this study. Therefore, fewer activated mononuclear cells in the male GT could account for the low SP/BP ratio of ZDV-TP. There are currently no data on intracellular drug concentrations in the GT of men with ongoing HIV replication or an active STD, however.
Differential phosphorylation between SP and BP may also be explained by diverse cell populations in the 2 compartments. The distribution of mononuclear cell subsets in semen is unknown. It has recently been observed in PBMCs that different cell types may phosphorylate nucleosides to varying extents, however.46
At the end of the 2-week treatment period, all subjects had undetectable HIV RNA concentrations in BP and all but 1 subject had undetectable HIV RNA in SP. It is notable that this subject had the lowest intracellular seminal exposure for ZDV-TP and 3TC-TP. Unfortunately, viral sequencing could not be performed on his samples.
The small number of men in this cohort precluded meaningful statistical evaluations of drug concentrations with HIV RNA and/or CD4 T-cell response. Other researchers have reported correlations between TP concentrations in PBMCs and drug efficacy,29 with higher TP concentrations leading to faster declines in HIV-1 RNA and quicker improvements in CD4 cell counts. Future PK-pharmacodynamic modeling with these data to explore the highly complex intracellular-extracellular drug concentration relation in BP and SP is planned but is outside the scope of this article.
The GT pharmacology data presented here are important for the health of the HIV-infected individual and to gain insight into the development of drug resistance in the GT. Understanding the pharmacology of nucleoside analogue reverse transcriptase inhibitors, the backbone of ARV therapy, is critical for choosing agents that minimize the development of resistance in the GT. Lower active ZDV concentrations in seminal cells may provide a pharmacologic basis for the male GT as a sanctuary site for HIV replication. Sexual transmission of resistant HIV is becoming increasingly common as ARV treatment becomes more widely available,19,47-53 and the choice of drug regimen clearly drives HIV resistance patterns in the male and female GT compartments.54-56 In addition to these seminal TP data for 3TC and ZDV, tenofovir diphosphate concentrations in seminal mononuclear cells have recently been reported to be 5 times higher than in PBMCs at steady state.34 The ability of tenofovir diphosphate to be equally phosphorylated in activated and quiescent cells may partially explain the high intracellular concentrations,44 although other factors currently under investigation may contribute to these differences.
For prevention of HIV transmission, these data are most critical for elucidating the influence of local drug concentrations on HIV shedding and the development of drug resistance. To assess the ability of drugs to act locally and protect HIV-negative individuals from infection, tissue concentrations of ARVs in the rectum, penile foreskin, and vaginal/cervical mucosa would be the most relevant to evaluate.
HIV is found in all male GT compartments, including organs that contribute to the generation of semen (ie, prostate, seminal vesicles, testicles). Some differences may be observed in the penetration of ARVs into each of these compartments, but they are unlikely to be clinically relevant.57 Recent developments in techniques to measure the fraction of the total drug concentration contributed by seminal vesicle-derived fluid and prostatic fluid should continue to advance the understanding of GT pharmacology and implications for local drug resistance.58
In conclusion, this is the first study to report ZDV, ZDV-TP, 3TC, and 3TC-TP exposures over a dosing interval in SP and seminal mononuclear cells in comparison to BP and PBMCs. These data have implications for development of compartmental resistance and further assist us in understanding ARV activity in the male GT.
The authors thank the study volunteers and the staff of the UNC Infectious Diseases Clinic, the Verne S. Caviness General Clinical Research Center, and the UNC Center for AIDS Research Virology and Clinical Pharmacology and Analytical Chemistry Cores.
1. WHO/UNAIDS. AIDS epidemic update. December 2006. Available at: http://www.unaids.org
. Accessed June 14, 2007.
2. Royce RA, Sena A, Cates W, et al. Sexual transmission of HIV
. N Engl J Med
3. Quinn TC, Weaver MJ, Sewankambo N, et al. Viral load and heterosexual transmission of human immunodeficiency virus type 1. Rakai Project Study Group. N Engl J Med
4. Chakraborty H, Sen PK, Helms RW, et al. Viral burden in genital secretion determines male-to-female sexual transmission of HIV
-1: a probabilistic empiric model. AIDS
5. Galvin SR, Cohen MS. The role of sexually transmitted disease in HIV
transmission. Nature Microbiology Reviews
6. Welzel TM, Gao X, Pfeiffer RM, et al. HLA-B Bw4 alleles and HIV
-1 transmission in heterosexual couples. AIDS
7. Winchester R, Pitt J, Charurat M, et al. Mother-to-child transmission of HIV
-1: strong association with certain maternal HLA-B alleles independent of viral load implicates innate immune mechanisms. J Acquir Immune Defic Syndr
8. Rietmeijer CA, Krebs JW, Feorino PM, et al. Condoms as physical and chemical barriers against human immunodeficiency virus. JAMA
9. National Institute of Allergy and Infectious Diseases. Workshop Summary: Scientific Evidence on Condom Effectiveness for Sexually Transmitted Disease (STD) Prevention
. Bethesda, MD: National Institutes of Health, National Institute of Allergy and Infectious Diseases; 2001.
10. Cates W Jr. The NIH condom report: the glass is 90% full. Fam Plann Perspect
11. Vernazza PL, Troiani L, Flepp MJ, et al. Potent antiretroviral treatment of HIV
-infection results in suppression of the seminal shedding of HIV
. The Swiss HIV
Cohort Study. AIDS
12. Cu-Uvin S, Caliendo A, Reinert S, et al. Effect of highly active antiretroviral therapy
on cervicovaginal HIV
-1 RNA. AIDS
13. Kotler DP, Shimada T, Snow G, et al. Effect of combination antiretroviral therapy
upon rectal mucosal HIV
RNA burden and mononuclear cell apoptosis. AIDS
14. Pereira AS, Kashuba ADM, Fiscus SA, et al. Nucleoside analogues achieve high concentrations in seminal plasma: relationship between drug concentration and virus burden. J Infect Dis
15. Tachet A, Dulioust E, Salmon D, et al. Detection and quantification of HIV
-1 in semen: identification of a subpopulation of men at high potential risk of viral sexual transmission. AIDS
16. Eron JJ, Smeaton LM, Fiscus SA, et al. The effects of protease inhibitor therapy on human immunodeficiency virus type 1 levels in semen (AIDS Clinical Trials Group protocol 850). J Infect Dis
17. Reddy YS, Gotzkowsky SK, Eron JJ, et al. Pharmacokinetic and pharmacodynamic investigation of efavirenz in the semen and blood of human immunodeficiency virus type 1-infected men. J Infect Dis
18. Coombs RW, Speck CE, Hughes JP, et al. Association between culturable human immunodeficiency virus type 1 (HIV
-1) in semen and HIV
-1 RNA levels in semen and blood: evidence for compartmentalization of HIV
-1 between semen and blood. J Infect Dis
19. Smith DW, Wong JK, Shao H, et al. Long-term persistence of transmitted HIV
drug resistance in male genital tract
secretions: implications for secondary transmission. J Infect Dis
20. Eron JJ, Vernazza PL, Johnston DM, et al. Resistance of HIV
-1 to antiretroviral agents in blood and seminal plasma: implications for transmission. AIDS
21. Si-Mohamed A, Kazatchkine M, Heard I, et al. Selection of drug-resistant variants in the female genital tract
of human immunodeficiency virus type 1-infected women receiving antiretroviral therapy
. J Infect Dis
22. Krieger JN, Coombs RW, Collier AC, et al. Intermittent shedding of human immunodeficiency virus in semen: implications for sexual transmission. J Urol
23. Sadiq ST, Taylor S, Kaye S, et al. The effects of antiretroviral therapy
-1 RNA loads in seminal plasma in HIV
-positive patients with and without urethritis. AIDS
24. Dumond JB, Yeh RF, Patterson KB, et al. Antiretroviral drug exposure in the female genital tract
: implications for oral pre- and post-exposure prophylaxis. AIDS
25. U.S. Department of Health and Human Services. DHHS Panel on Antiretroviral Guidelines for Adults and Adolescents-A Working Group of the Office of AIDS Research Advisory Council. Guidelines for the use of antiretroviral agents in HIV
-1-infected adults and adolescents. January 29, 2008. Available at: www.aidsinfo.nih.gov
26. Pereira AS, Smeaton LM, Gerber JG, et al. The pharmacokinetics
of amprenavir, zidovudine, and lamivudine in the genital tracts of men infected with human immunodeficiency virus type 1 (AIDS Clinical Trials Group Study 850). J Infect Dis
27. Anderson PL, Noormohamed SE, Henry K, et al. Semen and serum pharmacokinetics
of zidovudine and zidovudine-glucuronide in men with HIV
-1 infection. Pharmacotherapy
28. Back DJ, Burger DM, Flexner CW, et al. The pharmacology of antiretroviral nucleoside and nucleotide reverse transcriptase inhibitors. J Acquir Immune Defic Syndr
29. Fletcher CV, Kawle SP, Kakuda TN, et al. Zidovudine triphosphate and lamivudine triphosphate concentration-response relationships in HIV
-infected persons. AIDS
30. Cao YJ, Ndovi TT, Pason TL, et al. Effect of semen sampling frequency on seminal antiretroviral drug concentrations. Clin Pharmacol Ther
. 2007; Oct 3 [Epub ahead of print].
31. Pereira AS, Kenney KB, Cohen MS, et al. Simultaneous determination of lamivudine and zidovudine in human seminal plasma using high-performance liquid chromatography and tandem mass spectrometry. J Chromatogr B Biomed Sci Appl
32. Rodriguez JF, Rodriguez JL, Santana J, et al. Simultaneous quantitation of intracellular zidovudine and lamivudine triphosphates in human immunodeficiency virus-infected individuals. Antimicrob Agents Chemother
33. Reddy YS, Gotzkowsky SK, Eron JJ, et al. Pharmacokinetic and pharmacodynamic investigation of efavirenz in the semen and blood of human immunodeficiency virus type 1-infected men. J Infect Dis
34. Vourvahis M, Tappouni H, Patterson K, et al. The pharmacokinetics
and viral activity of tenofovir in the male genital tract
. J Acquir Immune Defic Syndr
35. Ghosn J, Chaix ML, Peytavin G, et al. Penetration of enfuvirtide, tenofovir, efavirenz, and protease inhibitors in the genital tract
-1-infected men. AIDS
36. Taylor S, van Heeswijk RP, Hoetelmans RM, et al. Concentrations of nevirapine, lamivudine and stavudine in semen of HIV
-1-infected men. AIDS
37. van Praag RM, van Heeswijk RP, Jurriaans S, et al. Penetration of the nucleoside analogue abacavir into the genital tract
of men infected with human immunodeficiency virus type 1. Clin Infect Dis
38. Chaudry NI, Eron JJ, Naderer OJ, et al. Effects of formulation and dosing strategy on amprenavir concentrations in the seminal plasma of human immunodeficiency virus type 1-infected men. Clin Infect Dis
39. Solas C, Lafeuillade A, Halfon P, et al. Discrepancies between protease inhibitor concentrations and viral load in reservoirs and sanctuary sites in human immunodeficiency virus-infected patients. Antimicrob Agents Chemother
40. Min SS, Corbett AH, Rezk N, et al. Protease inhibitor and nonnucleoside reverse transcriptase inhibitor concentrations in the genital tract
-1-infected women. J Acquir Immune Defic Syndr
41. Moore KHP, Barrett JE, Shaw S, et al. The pharmacokinetics
of lamivudine phosphorylation in peripheral blood mononuclear cells from patients infected with HIV
42. Anderson PL, Kakuda TN, Kawle S, et al. Antiviral dynamics and sex differences of zidovudine and lamivudine triphosphate concentrations in HIV
-infected individuals. AIDS
43. Gao WY, Shirasaka T, Johns DG, et al. Differential phosphorylation of azidothymidine, dideoxycytidine, and dideoxyinosine in resting and activated peripheral blood mononuclear cells. J Clin Invest
44. Shirasaka T, Chokekijchai S, Yamada A, et al. Comparative analysis of anti-human immunodeficiency virus type 1 activities of dideoxynucleoside analogs in resting and activated peripheral blood mononuclear cells. Antimicrob Agents Chemother
45. Gao W, Agbaria R, Driscoll JS, et al. Divergent anti-human immunodeficiency virus activity and anabolic phosphorylation of 2′,3′-dideoxynucleoside analogs in resting and activated human cells. J Biol Chem
46. Anderson PL, Zhen J, King T, et al. Concentrations of zidovudine- and lamivudine-triphosphate according to cell type in HIV
-seronegative adults. AIDS
47. Kroodsma KL, Kozal KA, Winters MA, et al. Detection of drug resistance mutations in the human immunodeficiency virus type 1 (HIV
-1) pol gene: differences in semen and blood HIV
-1 RNA and proviral DNA. J Infect Dis
48. Hecht FM, Grant RM, Petropoulos CJ, et al. Sexual transmission of an HIV
-1 variant resistant to multiple reverse-transcriptase and protease inhibitors. N Engl J Med
49. Yerly S, Kaiser L, Race E, et al. Transmission of antiretroviral-drug-resistant HIV
-1 variants. Lancet
50. Boden D, Hurley A, Zhang L, et al. HIV
-1 drug resistance in newly infected individuals. JAMA
51. Little SJ, Daar ES, D'Aquila RT, et al. Reduced antiretroviral drug susceptibility among patients with primary HIV
52. Weinstock HS, Zaidi I, Heneine W, et al. The epidemiology of antiretroviral drug resistance among drug-naive HIV
-1-infected person in 10 US cities. J Infect Dis
53. Wensing AM, van de Vijver DA, Angarano G, et al. Prevalence of drug-resistant HIV
-1 variants in untreated individuals in Europe: implications for clinical management. J Infect Dis
54. Neely MN, Benning L, Xu J, et al. Cervical shedding of HIV
-1 RNA among women with low levels of viremia while receiving highly active antiretroviral therapy
. J Acquir Immune Defic Syndr
55. Katzenstein D, Winters M, Fiscus S, et al, for the AIDS Clinical Trials Group 5077 Team. Drug resistance in plasma and genital compartments among viremic, multi-drug-experienced men and women [poster 618]. Presented at: 13th Conference on Retroviruses and Opportunistic Infections; Denver; 2006.
56. Kemal KS, Burger H, Mayers D, et al. HIV
-1 drug resistance in variants from the female genital tract
and plasma. J Infect Dis
57. Kashuba ADM, Dyer JR, Kramer LR, et al. Antiretroviral-drug concentrations in semen: implications for sexual transmission of human immunodeficiency virus type 1. Antimicrob Agents Chemother
58. Ndovi TT, Choi L, Carro B, et al. Quantitative assessment of seminal vesicle and prostate drug concentrations by use of a noninvasive method. Clin Pharmacol Ther